I. Field of the Invention
The present invention relates generally to improved formulations for the treatment of open evaporative cooling water systems and more specifically to formulations advantageous in the prevention of corrosion of zinc based coating materials of such systems during their normal operation.
II. Related Art
Open evaporative cooling water circulation systems are commonly employed in large industrial or commercial installations requiring large amounts of cooling water. In the operation of evaporative systems utilizing cooling water, amounts of cooling water are continually being evaporatively lost to the atmosphere, creating a need for quantities of additional or make-up water which, as required, are introduced and utilized at a rate dependent upon the service requirements of the installation. As introduced, make-up water commonly contains amounts of impurities and/or contaminants including dissolved gases, dissolved chemical compounds and suspended particulates. Since normal operation of these systems results in the consumption of reasonably large quantities of water, primarily through evaporation, over time this results in elevated levels of concentration of these contaminants and/or impurities. Of particular concern is the accumulation of carbonate alkalinity and the associated increase in pH levels.
Zinc based coatings are commonly employed on steel surfaces of open evaporative cooling equipment in order to offer corrosion inhibiting protection of the ferrous metal substrate. The well-known galvanized coatings are an example of such zinc based sacrificial coatings.
The elevation of carbonate alkalinity and pH levels during the normal operation of cooling water systems often results in the corrosion of the zinc based coatings of those systems. The corrosion of these coatings visually appears as a white, waxy, adherent deposit on the coating surfaces. This corrosion mechanism or syndrome is commonly referred to as "White Rust" by persons in the water treatment industries. White rust has been identified as a corrosion mechanism involving zinc metal and carbonate ion and resulting in the formation of the compound ZnCO.sub.3.3Zn(OH).sub.2.H.sub.2 O. White rust corrosion results in the loss of corrosion inhibition or protection of the ferrous metal substrate by the zinc coating and, if unchecked, may therefore lead to the premature failure of the cooling system equipment occasioned by the rapid anodic corrosion of zinc coated parts.
There are several current methods that have been employed or proposed for the prevention of white rust corrosion; they include the following techniques:
(a) The addition of sufficient quantity of an acid, most commonly sulfuric acid, to the cooling water in order to adjust the pH and prevent the presence or greatly reduce the concentration of carbonate ions in the cooling water has been suggested as a way to preclude the formation of zinc carbonate and thereby prevent or inhibit white rust corrosion. The addition of acid feed to cooling water systems, however, poses safety hazards to those personnel responsible for handling the acid and has the potential for aggressive corrosion of metals due to overfeed of acid to the system.
(b) Another approach suggests the addition of amounts of orthophosphate and/or zinc chemical compounds to the cooling water in order to provide a zinc-orthophosphate film capable of preventing white rust corrosion. Both the orthophosphate and zinc components are generally present in quantities of 20 to 100 mg/l as PO.sub.4 and as Zn. The Phosphate and Zinc-Phosphate treatments are effective as short-term treatments to provide corrosion preventing or passivating films on metallic surfaces.
The effectiveness of this approach to protection is usually short-lived, however, and the film soon degrades. Degradation of the passivating film must be followed by re-passivation to prevent localized white rust corrosion. Moreover, since the production of these passivating films is also dependent upon many influencing parameters of the cooling equipment, water and water treatments utilized, the quality and longevity of the passivating films remain as uncertain variables and the systems must be continually monitored for film failure and white rust formation.
Additional disadvantages of such passivating treatments include the possible formation of deposits on heat transfer surfaces resulting in decreased equipment efficiency, potential regulatory concerns relative to the cooling water disposal, potential for damage to the passivated film by oxidizing biocide treatments, by over feed of acidic pH control chemicals and/or by physical erosion.
The use of hard make-up water (water containing hardness ions, i.e., calcium and magnesium ions) is recommended in conjunction with current film-forming methods of white rust control as many of the film-forming corrosion inhibitors utilized require the incorporation of calcium ions in those films. Calcium ions present in the cooling water may also compete with zinc coatings for the carbonate ions in the cooling water thereby reducing the formation of zinc carbonate.
The requirement for hard water feed to cooling systems is disadvantageous in those instances where the available make-up water contains little or no calcium ions as supplied as well as in those instances where cooling make-up water is preferably softened in order to prevent calcium carbonate scale formation on heat transfer surfaces.
It will be appreciated that a benign approach to the reduction of white rust corrosion in zinc-based coatings which circumvents known disadvantages of previous techniques would be a welcome solution to a longstanding problem associated with water cooling systems. This is especially true of a cooling water treatment that would be compatible with the existing water conditions (i.e., relatively high pH and alkalinity and relatively low hardness).
Accordingly, it is a primary object of the present invention to provide a cooling water treatment system including formulations which are capable of preventing or significantly reducing white rust corrosion of zinc coatings utilized in cooling system equipment thereby extending the useful life of that equipment.
Another object of the invention is to provide a cooling water treatment system capable of preventing or significantly reducing white rust corrosion of zinc-based coatings on components in open-evaporative cooling water systems which avoids the prior limitations.
A further object of the present invention is to provide formulations effective in the prevention of white rust corrosion of zinc coatings in cooling systems with elevated carbonate alkalinity and pH values thereby eliminating the need for neutralizing acid feed to the cooling tower water and therefore avoiding the associated potential disadvantages.
A still further object of the present invention is to provide formulations effective in the prevention of white rust corrosion of zinc coatings in cooling water systems employing soft water (water essentially void of calcium or magnesium ions).
Yet another object of the invention is to provide formulations effective in the prevention of white rust corrosion of zinc coatings in cooling systems which employ chemical compounds which themselves will not contribute to scale, deposit or corrosion of the cooling water system when those formulations are utilized at residuals necessary to provide effective white rust control.
A yet still further object of the present invention is to provide formulations effective in the prevention of white rust corrosion of zinc coatings which are compatible with other chemical treatments utilized in cooling water systems for purposes of scale, deposit, microbiological fouling or corrosion inhibition.
Other and further objects of the present invention will become apparent to those skilled in the art upon a study of the following specification and appended claims.